Coastal Floodplain Environment

Coastal Floodplain Environment

During the Late Cretaceous (83-72 million years ago), Big Bend National Park was on the shoreline of the Western Interior Seaway. As the shoreline advanced and retreated, Big Bend experienced environments ranging from nearshore to continental environments. In these environments, dinosaurs, sharks, and other reptiles lived in the same region. Dinosaurs such as, Edmontonia and the horned Agujaceratops walked along the shore as the giant alligator, Deinosuchus, and the shark, Squalicorax, lurked in the waters below. The fossils found in these rocks have led to tremendous insight into the behavior of dinosaurs and the dynamics of this past coastal floodplain ecosystem. Barnum Brown’s discovery of Deinosuchus and the unearthing of an Agujaceratops herd are just a few of the amazing finds from this time in Big Bend.

Geological Setting

83 to 72 million years ago, Big Bend was near the shoreline of the Western Interior Seaway. The environment during this time resembled the tropical coastline of Mozambique in southwestern Africa. Because of many shoreline advances and retreats, this coastal floodplain environment varied from beaches, river deltas, and estuaries to coastal swamps and marshlands. This resulted in the complex geologic structure of the Aguja Formation, which contains a variety of rock types ranging from sandstones and mudstones to coal beds and mudstones with paleosols (ancient soils).

Block Diagram by Tom Lehman

The variety of fossils preserved in the Aguja Formation is another result of the extremely variable environment. Fossils include petrified woods; invertebrates such as bivalves, gastropods (snails), and cephalopods (octopuses, squids, ammonites, and belemnites); vertebrates such as sharks, fish, turtles, several dinosaurs and the gigantic alligatoroid, Deinosuchus (which you will see in the Gallery of Giants), as well as rare small mammal teeth.

Featured Fossils

Agujaceratops is a plant-eating horned dinosaur that could get up to 21 feet (7 meters) long. It used its large bony frill and horns to intimidate predators and rivals and for mating displays. A skull and isolated bones have been found at Big Bend.

Teratophoneus is a smaller cousin of the T. rex whose fossils are very rare. Teeth, limb and skull bones were found at Big Bend.

Since the Aguja Formation preserves the diversity in environmental settings, the fossils found here are equally as diverse and scientifically important. Below are the significant fossil occurrences at Big Bend in the Aguja Formation:

In 1938, William Strain, an instructor for the El Paso College of Mines, collected many fossil bone fragments in an attempt to find a complete dinosaur skeleton for the college. In 1982, then graduate student, Tom Lehman identified these fragments as a new dinosaur, Chasmosaurus (Agujaceratops) mariscalensis. Lehman also discovered that these bone fragments were from at least 20 individuals of variable ages. This discovery of a herd of Agujaceratops supports the hypothesis that dinosaurs congregated in herds, which is still debated to this day.

Schematic of Agujaceratops Herd Assemblage

In 2010, Tom Lehman discovered a new genus of side-necked turtle, Chupacabrachelys. Lehman also observed several interesting punctures and scratches in the turtle’s shell, most likely from a predator. The attacker was determined to be the giant alligator Deinosuchus based on the size of the turtle and the bite mark spacing. Even more, the wounds showed signs of healing indicating the turtle escaped being eaten. This sheds light on the incredible resilience of turtles that has allowed them to survive for over 200 million years.

Coprolites (fossilized dung) found in an aquatic bed of the Aguja Formation were determined to be fossilized shark dung based on the presence of undigested gar scales and the coiled or folded morphology of the dung. The presence of a specific shark’s teeth found at the site suggested that the dung was from Goblin Shark relative, Scapanorhynchus.

Most of the fossilized wood from Big Bend occurs as fallen logs, but some stumps have been found in their original growth locations. In fact, 18 stumps of 2 species of flowering tree (Metcalfeoxylon and Agujoxylon) were found. Based on a maximum stump diameter of 4.3 feet (1.3 meters), this “paleo-forest” of tropical evergreens would have had a canopy height of 131-164 feet (40-50 meters). The absence of dinosaur fossils in this area suggests that herbivorous dinosaurs were limited to habitats supporting plants with lower browsing heights.

Legendary paleontologists Barnum Brown (named the T. rex) and R. T. Bird discovered Deinosuchus in 1940 (before Big Bend was established). Brown based his reconstruction on that of a crocodile, which is inaccurate since Deinosuchus is now known to be a primitive alligatoroid. Since then, more specimens have been found including a complete skull to help create a more accurate reconstruction (on display at the Fossil Discovery Exhibit).

The presence of juvenile dinosaur bones at localities that contain palm seeds is significant because it suggests that baby dinosaurs used palms as cover and as high-energy food. The presence of egg-shell fragments near these sites in addition to the bones suggests that there is a link between nesting habitat preference and hatchling behavior, but no intact dinosaur nests have been found at Big Bend.

Places in the Park

Places in the park to see these Late Cretaceous coastal sediments include:

Badlands wayside by the Maverick (western) entrance station

SanVicente: Located on River Road East around the San Vicente Crossing

McKinney Hills: Located on Old Ore Road (4x4, high clearance only) between McKinney Springs and Roys Peak Vista Campsites

Fossil Discovery Exhibit

Thank you!

This exhibit was made possible by the donations of generous supporters of the Big Bend Conservancy. Their legacy will continue to provide visitors with a world class exhibit experience for years to come.